Known gas sensor arrangements comprise a radiation source emitting radiation, a gas measuring chamber, which can be filled with a sample gas that contains at least one analyte to be measured, and at least one detector device detecting radiation, which detector produces an output signal dependent on the presence and/or the concentration of the analyte. Gas sensor arrangements of this kind are known for the detection of a wide range of analytes, for example carbon dioxide or methane. Conventional gas sensors are based on the property of many polar gases to absorb radiation in the infrared (IR) wavelength range. The IR light is capable of causing excited states of the molecules by excitation of rotation and vibration oscillations by interacting with the dipole moment of the polar molecule. The thermal energy of the IR light is transferred to the gas in this way and in the same way the intensity of an IR ray passing through the gas volume is reduced. According to the excitation states, absorption occurs in a wavelength that is characteristic in each case of the gas in question, for example at 4.25 μm in the case of CO2.
The detection of carbon dioxide (CO2) in particular, is now becoming increasingly important in a large number of application areas. In the motor vehicle field, for example, carbon dioxide detection can be used to monitor the CO2 content of the interior air to increase energy efficiency in the case of heating and air conditioning, in order to initiate a supply of fresh air via suitable fan flap activation only if required, i.e. when there is an increased CO2 concentration. In addition, modern vehicle air conditioning systems are also based on CO2 as a coolant, so that CO2 gas sensors in the motor vehicle field can fulfill a monitoring function in connection with escaping CO2 in the event of any defects. In the motor vehicle field in particular, gas sensors must meet the highest requirements in respect of sturdiness, reliability and miniaturizability. In addition, the response time of the sensor may not exceed certain limit values for safety applications.
The output signal of known detector devices, such as those disclosed in German patent application DE 10 2005 055 869.7, can be described quite generally as a response to an abrupt change in concentration by an exponential function according to the following equation (1).
                              y          ⁡                      (            t            )                          =                              y            0                    ·                      (                          1              -                              ⅇ                                                      -                    t                                    τ                                                      )                                              (        1        )            
Here y(t) describes the detector signal emitted by the detector at a time t, y0 the final value to which the detector signal approximates and τ the time constant of the exponential function.
To characterize the response behavior of the gas sensor, it is not the time constant τ, which describes the tangent to the exponential function at zero, which is used for the most part in practice, but the so-called t90 time, which describes the time at which the detector signal has attained 90% of the final value y0. Usually a t90 time of less than 10 seconds is required for safety applications of a gas sensor.
However, various design boundary conditions that will be examined in greater detail below result in the t90 time often being too long in the case of known gas sensors.
For example, the use of a filter membrane on the inlet opening, as is necessary to protect against pollution, causes a slower response of the gas sensor, in principle due to the retarded diffusion inwards. The stringent demands made on miniaturization of the gas sensor, furthermore, reduce the dimensions of the inlet opening for the sample gas and likewise lead to increased response times.
To increase the sensitivity of the sensor, it is proposed in German patent application DE 10 2005 055 860.7 to use a rotationally symmetrical cell. However, this implies an extended gas mixing time and thus once more an increase in the response time. Thus known gas sensors have the disadvantage that the t90 time lays above the limit values required for safety applications.
On the other hand, the design conditions should be changed as little as possible, as otherwise, other disadvantages such as insufficient sensitivity have to be accepted.